1. Field of the Invention
The invention concerns a process for spreading the volume of traffic over different control channels of a radio transmission system, and apparatus for practicing such a process.
2. Description of the Prior Art
In a radio communications system (radio transmission system) known from DE-AS 27 33 503 the service area (the entire primary coverage area) is divided up into a large number of zones (radio zones) (cellular system) and each radio zone is allocated at least one control channel (organisation channel) and several traffic channels. In a radio zone with a large number of mobile telephone subscribers (mobile radio stations) the transmitter/receiver in the radio station (base) has to be provided with several control channels. To simplify the transmission and reception separation filter the transmission and reception frequency band is split up into an upper and a lower band. The mobile radio stations are divided up into at least two groups each of which uses one of the transmission and reception frequency bands. The division of the transmission and reception frequency band into two bands is chosen so that both groups of mobile radio stations share a common band. At least some of the control channels are located in this common band.
A mobile radio station of a first group selects a control channel, e.g. for the establishment of connections. To this end the mobile radio station scans the control channels allocated to the first group for one with a good signal-to-noise ratio. If the signal-to-noise ratio of the selected control channel deteriorates because of interference to the radio field propagation or for some other reason, then the mobile radio station, on receipt of a certain control channel code, can also use a control channel allocated to another group, e.g. the second group. This allocation of control channels of another group is only undertaken in a radio zone if it has a small number of mobile radio stations.
If there are several control channels available in a radio zone, then the volume of traffic corresponding to the mobile radio stations located and registered as being in this radio zone is spread over the control channels of this radio zone. In the known radio transmission system (DE-AS 27 33 503) the mobile radio stations are split up into groups to reduce the cost of the transmitter and receiver separation filter. Regard for the traffic situation, in particular a system of control by spreading the volume of traffic over different control channels, is neither anticipated nor mentioned. If four control channels are allocated to a radio zone, for example, and if there are in that radio zone only two out of a total of four groups of mobile radio stations, then the volume of traffic in the zone is spread over two control channels allocated to only the two groups of mobile radio stations.
If the number of control channels in a radio zone belonging to that radio zone or the frequency and consequently the channel number of a control channel changes, then the mobile radio stations should automatically and independently assign themselves to the thus formed new set of control channels. If, for example, a new control channel is allocated to the radio zone, then everything possible should be done to prevent the already existing control channels and the new control channel from becoming temporarily overloaded in the short term by a large number of individual transfers of mobile radio stations.
The problem underlying the invention was to devise a process for spreading the volume of traffic over different control channels of a radio zone. In the event of a change in configuration, i.e. in the event of a control channel failure, the mobile radio stations should require no individual change commands to a new control channel.